Terrapins fight solo or in groups of two to four and attack their opponents by punching, which does not do much damage. They patrolled the halls of Bowser's Keep in search of intruders, and appear mostly in the very beginning of the game, before Exor crashes into Bowser's Keep. They only had 10 HP and 1 attack power, and 8 defense; this made battling them good training for Mario at that point in the game. Mario also gains no EXP for defeating them.

Terrapins are the precursors to the Koopatrols of the Paper Mario series. However, Terrapins and Koopatrols themselves are very different from each other in stats, although their main purposes are identical in that they are both armor-clad Koopas who guard Bowser's Castle.